The present invention relates generally to the analysis of electrical networks, and more particularly, to the analysis of DC electrical load flow.
In large manufacturing or process facilities, electrical power is often provided by diverse, AC and DC networks. In a nuclear power plant, for example, a DC power supply operates equipment and components, during both normal and accident conditions. A large, for example, 60 cell station battery, provides DC power to a DC distribution panel. During normal operation, the DC panel provides power to the station computer, AC switchgear, motor operated valves, AC/DC inverter equipment and similar components. Control power to operate the Diesel Generator and associated breakers is supplied from the DC system.
DC system reliability is of growing concern, as loads continue to be added to existing systems. The loads being supplied from DC systems are often critical loads for which operability is sensitive to the supply voltage. For these reasons, it is important to perform load flow, voltage drop and short circuit analysis on these DC systems to verify system adequacy and enable the user to know where any system limitations exist.
Conventionally, DC network analysis was performed using simplified modeling techniques. Load currents were calculated based on a single assumed value of the voltage, dependent on the load type. One problem with this approach, is that it assumes a static voltage and thus a static current value for the loads during the entire battery discharge time. In reality, the system voltage typically changes with time, by 10% to 25%. As the voltage changes, the load currents are also changing. For constant impedance loads (lights, relays, etc.) the current decreases as the voltage decreases. For constant power loads (inverters, motors, etc.) the current increases as the voltage decreases.
In one form of conventional modeling, an analytical tool which is designed for an analysis of an AC network, is "fooled" so as to appear as a DC network. To this extent, DC models have utilized significant simplifications resulting in limited accuracy. For example, in tools originally designed to analyze a three phase AC power distribution network, the model was "fooled" by, for example, setting the power factor to 1.0, and dividing the results by the square root of 3. These models suffered from the further deficiencies noted above, of failing to accommodate the time dependence of the source voltages.
Another known model is based on analysis using a voltage divider network. Instead of explicitly analyzing the entire network, the network is simplified to a few branches which are explicitly analyzed, and conditions for the individual components are estimated. This also results in forcing the program/user to utilize separate networks for load flow and fault current calculations.
The prior analytical tools are typically implemented by custom computer programs written in a language such as BASIC, FORTRAN or C. These do not permit the user to compute and visually comprehend, the time dependent affects on the network. This time dependence can simply be a result of the battery being drawn down, with or without changes in the state of elements in the network. For example, many DC loads in the plant are subject to sequencing, with rapid switching between on and off, over a period ranging from milliseconds to minutes. Moreover, certain sequences of load activation are assumed to occur during various fault or accident scenarios. Conventional analytical tools are not adapted to handle the time dependence of the voltage source, with the ability to analyze a comprehensive sequence of constant power and/or constant impedence type loading during such time.
As a result, the conventional techniques for analyzing DC electrical load flow, may indicate that a particular network is either overly conservative or has insufficient margin relative to the licensing or other margin requirements, when in fact the system is quite satisfactory.